The invention relates to a method for generating a periodic circular structure in a basic support material, such as for patterning a basic layer for the magnetic bits of a magnetic disk media.
Magnetic recording devices in the form of circular discs coated uniformly with thin films of magnetic materials are used in many information systems. The data is recorded as magnetic bits in the thin film media. A magnetic bit means an area of the film that has been mainly magnetized in a direction by a writing head. The bits are read by a reading head which is sensitive to the magnetization orientation of the bits. The disc rotates usually around an axis perpendicular to the surface of the disc. During the rotation the write and/or read operations are performed. Therefore, the magnetic bits are positioned on circular tracks around the rotational axis.
The areal density of data stored on this kind of magnetic storage disks (i.e. so-called hard disks) has increased with an average annual growth range of about 60 to 100% each year since the early 90's. This tremendous growth rate allowed lower costs per stored bit and higher speed of data access. Storage devices having an areal density of about 60 Gbits/inch2 are expected to be offered in the market in the near future, i.e. second half of 2003.
Consequently, the increase of the areal density can only be achieved by a decrease of the size of a magnetic bit cell. For example, a 100 Gbits/inch2 generation will require a size of a magnetic bit cell to be approximately 80×80 nm2 assuming a square shaped bit cell. The magnetic bit cell in a thin film crystalline media may contain several tens to hundreds of magnetic grains. A respectable number of grains is required in order to allow an acceptable signal to noise ratio for each magnetic bit cell. Therefore, as far as the size of the magnetic bit cell has to decrease for the sake of higher areal densities, the size of grains must become smaller as well. However, the level of minimization will touch physical limits due to the so-called super-paramagnetic limit which is touched when the size of grains become too small to retain its magnetization at room temperature for a sufficient period of time.
A potential solution for the problem is to use instead of a continuous thin film media a patterned media where each magnetic bit cell is formed as a lithographically defined magnetic element. These elements behave as single oriented magnetic domains being stable at room temperature. Moreover, each of these lithographically defined magnetic elements can be individually switched and read. An added advantage is that the transitions from one magnetic state to another one are much sharper than those in a continuous medium. It is required to arrange these magnetic elements in periodic arrays to be synchronized with the read/write signal.
A major obstacle to the realization of a patterned magnetic storage media is the difficulty of a mass production of a suitable patterned substrate at an acceptable cost. The considerably high areal density above 100 Gbits/inch2 requires bit sizes to be well below 100 nm in length and/or width. Therefore, in a suitable production technique, large areas have to be patterned having a comparably high resolution of the periodic structure at acceptable throughput and cost. Electron beam lithography is capable of producing such patterns but the throughput is far from being acceptable. Laser interference lithography has long been regarded as a possible technique as it can create periodic patterns over comparably large areas. However, up to now interference lithography has been shown to create only linear periodic structures such as arrays of lines or arrays of dots on square, rectangular or hexagonal grids.